Image processing method and apparatus

ABSTRACT

It is difficult to adjust a black plate while maintaining accurate color reproduction in a color printer that forms an image using C, M, Y, and K color agents. 
     Upon generating a conversion condition used to convert a signal on a uniform color space into a CMYK signal which depends on a printer, the signal on the uniform color space according to a colorimetric value is temporarily converted into an RGB signal, and the RGB signal is then converted into a CMYK signal. After that, a black amount adjustment unit ( 2104 ) adjusts a K value using a black amount adjustment function based on a user&#39;s instruction.

FIELD OF THE INVENTION

The present invention relates to an image processing method andapparatus and, for example, to a color reproduction process of aprinter.

BACKGROUND OF THE INVENTION

As a method of making color correction for improving a colorreproduction effect in a color reproduction process of a printer orprinting press, a color masking method of obtaining data on an outputcolor space by making matrix operations of data on an input color space,a method of converting data on an input color space into data on anoutput color space using a lookup table (LUT), and the like areprevalently used.

However, the output characteristics of a color printer or printing pressexhibit strong nonlinearity. Therefore, in a global method such as thecolor masking method, i.e., a color correction method in which a changein element of a matrix influences the overall output color space, thecharacteristics of a color printer or printing press cannot besatisfactorily approximated in the entire color gamut. Also, in themethod using a LUT, table values are normally obtained by the colormasking method, and difficulty in color reproduction remains unsolved.

In general, most of color printers form images using four, i.e., C, M,Y, and K color agents. Images formed by these four color agents lookdifferent depending on the way black (K) is generated, although theyexhibit the same colorimetric values. Various black generation methodsare known. However, most of these methods must change conditionsdepending on the types of images, the tone characteristics of a printer,and the like, and it is difficult to easily adjust the way black isgenerated.

SUMMARY OF THE INVENTION

The present invention has been proposed to solve the conventionalproblems, and has as its object to provide a profile which canaccurately approximate strong nonlinear output characteristics of acolor printer or printing press, and allows accurate color reproduction.

It is another object of the present invention to realize flexibleadjustment of a black plate while maintaining accurate colorreproduction in a color printer which forms an image using C, M, Y, andK color agents.

It is still another object of the present invention to generate aconversion table used to realize accurate color reproduction in variouscolor printers by absorbing differences in characteristics forrespective printers.

As a method of achieving the above object, an image processing method ofthe present invention comprises the following steps.

That is, there is provided an image processing method comprising theinput step of inputting colorimetric values of color patches output froman output device, the first generation step of generating, on the basisof the colorimetric values, a first conversion condition used to converta signal on a first color space which depends on a target device into asignal on a second color space which is independent from a device, andthe second generation step of generating, on the basis of thecolorimetric values, a second conversion condition used to convert asignal on the second color space into a signal on a third color spacewhich depends on the output device, the second generation stepgenerating the second conversion condition by including the first stepof converting a signal on the second color space into a signal on an RGBcolor space, the second step of converting a signal on the RGB colorspace into a signal on the third color space, and the black amountadjustment step of adjusting a black color value in the signal on thethird color space.

Also, there is provided an image processing method of generating aconversion condition between a signal on a device-independent uniformcolor space, and a signal on a CMY color space depending on a device, onthe basis of colorimetric values of color patches output from thedevice, comprising the step of generating the conversion condition toadjust a K value upon temporarily converting the signal on the uniformcolor space according to the colorimetric value into a signal on an RGBcolor space, and converting the signal on the RGB color space into aCMYK signal on the CMY color space.

Furthermore, there is provided an image processing method whichcomprises the steps of inputting colorimetric values of patches, whichare generated by a target device, on the basis of device-dependent colordata which is specified by a plurality of color component data includinga black component, generating, on the basis of the colorimetric values,a first lookup table used to convert device-dependent color data, whichis specified by a plurality of color component data that do not includea black component, into device-independent color data, and generating,by looking up the first lookup table, a second lookup table used toconvert the device-independent color data into the device-dependentcolor data, which is specified by the plurality of color component datathat include the black component, the method further comprising thesteps of inputting a print agent total amount condition used to controla total amount of print agents, and a black amount adjustment conditionthat pertains to a black component data generation method, andgenerating the first and second lookup tables using a conversioncondition, which is obtained from the print agent total amount conditionand the black amount adjustment condition, and is used to convert thedevice-dependent color data which is specified by the plurality of colorcomponent data that do not include the black component into thedevice-dependent color data which is specified by the plurality of colorcomponent data that include the black component.

That is, there is provided an image processing method comprising thecolorimetry step of inputting colorimetric values of color patchesoutput from an output device, the first generation step of generating,on the basis of the colorimetric values, a first conversion conditionused to convert a signal on a first color space which depends on atarget device into a signal on a second color space which is independentfrom a device, and the second generation step of generating, on thebasis of the colorimetric values, a second conversion condition used toconvert a signal on the second color space into a signal on a thirdcolor space which depends on the output device, the second generationstep including the step of generating the second conversion condition bytemporarily converting a signal on the second color space into a signalon an RGB color space, and then converting the signal on the RGB colorspace into a signal on the third color space.

For example, the second generation step includes the step of generatingan RGB conversion condition used to convert a signal on the second colorspace into a signal on the RGB color space.

There is provided an image processing method of generating a conversioncondition between a signal on a device-independent uniform color space,and a signal on an RGB color space depending on a device, on the basisof colorimetric values of color patches output from the device,comprising the step of analyzing a distribution of the colorimetricvalues on the uniform color space, and determining a parameter upongenerating the conversion condition on the basis of the analysis result.

For example, the parameter is a gamma value used in gamma conversion fora signal on a CMY color space obtained by converting a signal on the RGBcolor space.

As a means for achieving the above objects, an image processingapparatus of the present invention has the following arrangement.

That is, there is provided an image processing apparatus which has firstconversion means for converting a signal on a first color space, whichdepends on a target device, into a signal on a second color space, whichis independent of a device, and second conversion means for converting asignal on the second color space into a signal on a third color spacewhich depends on an output device, the apparatus comprising colorimetrymeans for inputting colorimetric values of color patches output from theoutput device, first generation means for generating, on the basis ofthe colorimetric values, a first conversion condition to be looked up bythe first conversion means, and second generation means for generating,on the basis of the colorimetric values, a second conversion conditionto be looked up by the second conversion means, wherein the secondgeneration means generates the second conversion condition bytemporarily converting a signal on the second color space into a signalon the RGB space, and then converting the signal on the RGB space into asignal on the third color space.

The present invention comprises the following arrangement as a means forachieving the above objects.

An image processing method according to the present invention isdirected to an image processing method of generating, a printer modelused to convert device-dependent data into device-independent data, onthe basis of colorimetric results of color patches output from an outputdevice, and generating, a conversion table used to convert thedevice-dependent data into the device-independent data, on the basis ofthe printer model, comprising the steps of obtaining tonecharacteristics of the output device from the colorimetric results, andcalculating a conversion condition used to convert the tonecharacteristics into linear characteristics, converting the tonecharacteristics of the output device into linear characteristic bymaking tone conversion using the conversion condition, and generatingthe printer model on the basis of colorimetric results corresponding todata converted by tone conversion.

An image processing apparatus according to the present invention isdirected to an image processing apparatus for generating, a printermodel used to convert device-dependent data into device-independentdata, on the basis of colorimetric results of color patches output froman output device, and generating, a conversion table used to convert thedevice-dependent data into the device-independent data, on the basis ofthe printer model, comprising calculation means for obtaining tonecharacteristics of the output device from the colorimetric results, andcalculating a conversion condition used to convert the tonecharacteristics into linear characteristics, tone conversion means forconverting the tone characteristics of the output device into linearcharacteristic by making tone conversion using the conversion condition,and generation means for generating the printer model on the basis ofcolorimetric results corresponding to data converted by tone conversion.

Other features and advantages of the present invention will be apparentfrom the following description taken in conjunction with theaccompanying drawings, in which like reference characters designate thesame or similar parts throughout the figures thereof.

BRIEF DESCRIPTION OF THE DRAWINGS

The accompanying drawings, which are incorporated in and constitute apart of the specification, illustrate embodiments of the invention and,together with the description, serve to explain the principles of theinvention.

FIG. 1 is a block diagram showing an example of the arrangement of animage processing apparatus according to the first embodiment;

FIG. 2 shows an example of an RGB→Lab conversion table;

FIG. 3 is a flow chart showing the sequence for executing device RGB→Labconversion by obtaining correspondence between device RGB values

Lab colorimetric values;

FIG. 4 shows an example of a sample image;

FIG. 5 shows an example of colorimetry results of a color patchcolorimetry unit;

FIG. 6 is a view for explaining selection of sample points;

FIG. 7 is a graph for explaining a weighting function according todistance d;

FIG. 8 is a graph for explaining a function for changing the number ofsample points;

FIG. 9 is a block diagram showing an example of the arrangement of animage processing apparatus according to the second embodiment;

FIG. 10 is a block diagram showing an example of the arrangement of animage processing apparatus according to the third embodiment;

FIG. 11 is a flow chart showing the flow of a device RGB→CMYK conversionprocess according to the fourth embodiment;

FIG. 12 shows an example of color patch colorimetry results in thefourth embodiment;

FIG. 13 is a block diagram showing an example of the arrangement of animage processing apparatus according to the fifth embodiment;

FIG. 14 is a block diagram showing an example of the arrangement uponimplementing the fifth embodiment on a computer system;

FIG. 15A is a block diagram showing the detailed operation of a patchgeneration/colorimetry system and CMYK→Lab conversion LUT generation inthe fifth embodiment;

FIG. 15B is a block diagram showing the detailed operation of deviceRGB→Lab conversion LUT generation and Lab→CMYK conversion LUT generationin the fifth embodiment;

FIG. 16 shows an example of a GUI in the fifth embodiment;

FIG. 17A is a flow chart showing a patch output process of the fifthembodiment;

FIG. 17B is a flow chart showing a colorimetry process of the fifthembodiment;

FIG. 17C is a flow chart showing a CMYK→Lab conversion LUT generationprocess of the fifth embodiment;

FIG. 17D is a flow chart showing a Lab→CMYK conversion LUT generationprocess of the fifth embodiment;

FIG. 17E is a flow chart showing a device RGB→Lab conversion LUTgeneration process of the fifth embodiment;

FIG. 18 is a block diagram showing the arrangement of an imageprocessing apparatus according to the sixth embodiment;

FIG. 19 is a block diagram showing the detailed arrangement of acolorimetric value distribution analyzer of the sixth embodiment;

FIG. 20 is a flow chart showing a device RGB→Lab conversion LUTgeneration process upon executing an Lab→CMYK conversion LUT generationprocess of the fifth embodiment;

FIG. 21 is a flow chart showing a gamma value calculation method basedon colorimetric value distribution analysis of the sixth embodiment;

FIG. 22 is a graph showing the relationship between a gray signal and L*value;

FIG. 23 is a block diagram showing the detailed arrangement of a deviceRGB→CMYK converter in the seventh embodiment;

FIG. 24 is a graph showing an example of a black amount adjustmentfunction in the seventh embodiment;

FIG. 25 is a flow chart showing a black amount adjustment process in theseventh embodiment;

FIG. 26 shows an example of a GUI in the seventh embodiment;

FIG. 27 is a graph showing an example of the CMYK value−densitycharacteristics of an output device;

FIG. 28 shows a printer model example of the output device shown in FIG.27;

FIG. 29 shows an example of a printer model obtained by an output devicewith linear CMYK−density characteristics;

FIG. 30 is a block diagram showing an example of the arrangement of adevice RGB→CMYK converter according to the eighth embodiment;

FIG. 31 is flow chart showing the flow of the process of a linearitycorrection LUT generator;

FIG. 32 shows an example of a CMYK single color value−CMYK single colordensity table; and

FIG. 33 is a graph for explaining a linearity correction LUT.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

Preferred embodiments of the present invention will now be described indetail in accordance with the accompanying drawings.

First Embodiment

FIG. 1 is a block diagram showing an example of an image processingapparatus of this embodiment.

A signal input to the image processing apparatus shown in FIG. 1 is animage signal of a color space depending on an arbitrary device and, forexample, can be an RGB signal which represents an image scanned from adocument by an arbitrary scanner, or a CMYK signal to be output to anarbitrary printer. When this embodiment is applied to a copying machine,an input signal is an RGB signal that represents an image scanned by ascanner. For the purpose of proof (test print, proof for correction),the input signal is a CMYK signal to be output to a printing press as atarget.

Such input signal is input to an input color→Lab converter 101, and isconverted into a signal on an Lab color space as a device-independentcolor space. This conversion is implemented by LUT conversion using aninput color→Lab conversion LUT 102.

As tables of the input color→Lab conversion LUT 102, a tablecorresponding to the color space of the input signal must be set. Forexample, when an image signal depending on an RGB color space of scannerA is input, a three-dimensional input−three-dimensional output RGB→Labconversion table which represents correspondence between RGB values thatdepend on the RGB color space of scanner A, and Lab values, is set as atable of the input color→Lab conversion LUT 102. Likewise, when an imagesignal which depends on a CMYK color space of printer B is input, afour-dimensional input−three-dimensional output CMYK→Lab conversiontable which represents correspondence between CMYK values that depend onthe color space of printer B, and Lab values, is set as a table of theinput color→Lab conversion LUT 102.

FIG. 2 shows an example of an RGB→Lab conversion table, and showscorrespondence between 8-bit RGB and Lab values. Since an actual tablestores Lab values using typical RGB values as addresses, the inputcolor→Lab converter 101 extracts an Lab value near the input RGB valuefrom the table, and makes an interpolation operation for the extractedLab value to acquire an Lab value corresponding to the input RGB value.

An Lab signal output from the input color→Lab converter 101 is convertedinto a signal of a device RGB color space by an Lab→device RGB converter104 on the basis of a device RGB→Lab conversion LUT 105. Details of thisconversion process will be described later.

If the color space of the input signal is an RGB color space, its colorgamut is often broader than the color reproduction range of a printer.For this reason, the Lab signal output from the input color→Labconverter 101 is mapped on the color reproduction range of a printer 107by a color space compression converter 103 (gamut mapping), and is theninput to the Lab→device RGB converter 104. As a practical method ofgamut mapping, a method of executing a color space compression processin a uniform color space, as disclosed in Japanese Patent Laid-Open No.8-130655, or the like may be used. Also, other known color spacecompression methods may be used.

A signal of the device RGB color space output from the Lab→device RGBconverter 104 is converted into a signal of a CMYK color space, whichdepends on the printer 107, by a device RGB→CMYK converter 106, and theconverted signal is sent to the printer 107. Various methods areavailable for RGB→CMYK conversion, and an arbitrary method may be used.For example, conversion formulas (1) below are used.C=(1.0−R)−KM=(1.0−G)−KY=(1.0−B)−KK=min{(1.0−R), (1.0−G), (1.0−B)}  (1)[Lab→Device RGB Conversion]

Details of the Lab→device RGB converter 104 will be explained below.

The Lab→device RGB converter 104 converts a signal on the basis ofcorrespondence between device RGB values and Lab colorimetric values,which is obtained in advance. FIG. 3 is a flow chart showing thesequence for executing device RGB→Lab conversion by obtainingcorrespondence between device RGB values

Lab colorimetric values. Of course, if the correspondence between deviceRGB values

Lab colorimetric values has already been obtained, steps S1 and S2 areskipped.

Step S1

A color patch generator 108 generates sample images formed by aplurality of color patches, as shown in FIG. 4. The RGB signals of thegenerated sample images are output to the printer 107 via the deviceRGB→CMYK converter 106 to obtain sample images 109.

The color patch generator 108 generates sample images by equallydividing the device RGB color space. In the example of FIG. 4, the RGBcolor space in which R, G, and B data are respectively expressed by 8bits is equally divided into 9×9×9 to obtain 729 patches. Generally, acolor space depending on the printer 107 is a CMYK color space, butsince the RGB color space can be converted into the CMYK color space viaa conversion rule, the RGB color space is considered as a color spacedepending on the printer 107.

Step S2

A color patch colorimetry unit 110 measures the color patches of theobtained sample image 109 to obtain Lab colorimetric values of therespective color patches. The obtained Lab colorimetric values aredistributed on an Lab color space, as shown in FIG. 5. With thisoperation, the RGB values generated by the color patch generator 108 andLab colorimetric values measured by the color patch colorimetry unit 110can be obtained and, hence, a table of the device RGB→Lab conversion LUT105 can be obtained. Using this device RGB→Lab conversion LUT 105,Lab→device RGB conversion is done.

When an LUT is used, an interpolation operation such as cubicinterpolation, tetrahedral interpolation, or the like as a known methodis used. Such interpolation operation requires grids at equal intervals,which correspond to the input side of an LUT. In the table of the deviceRGB→Lab conversion LUT 105, device RGB values are arranged at equalintervals, but Lab colorimetric values are not arranged at equalintervals. For this reason, when Lab values are to be input, the tableof the device RGB→Lab conversion LUT 105 does not form an LUT havinggrids at equal intervals. Hence, an interpolation operation that inputsLab values cannot be simply made. Hence, Lab→device RGB conversion ismade in the following sequence.

Step S3

Distances d (equivalent to a color difference based on the Lab colordifference method) between Lab values included in the table of thedevice RGB→Lab conversion LUT 105 and input Lab values are calculatedand are stored in a memory.

Step S4

As shown in FIG. 6, N entries (●) are selected in ascending order ofdistance d with respect to the input Lab values (⊚). At this time,entries are described as follows in ascending order of distance d.

RGB value Lab colorimetric value Distance RGB₁ Lab₁ d₁ RGB₂ Lab₂ d₂ RGB₃Lab₃ d₃ . . . . . . . . . RGB_(N) Lab_(N) d_(N)For d₁<d₂<d₃<. . . <d_(N).

Step S5

A converted value (RGB value) corresponding to an input Lab value iscalculated by:RGB=(1/N)×Σ_(i=1) ^(N) RGBi×f(di)for f(x)=1/(1+x ⁴)

Since a function f(x) has characteristics, as shown in FIG. 7, thecalculation given by the above equation makes an interpolation operationby multiplying an RGB value corresponding to a closer Lab colorimetricvalue on the Lab color space by a larger weighting coefficient.

The number N of sample points used in the interpolation operation can bedefined by a constant (e.g., 8) in the entire Lab color space. However,since colorimetric values are concentrated on a region of low lightnessL*, as shown in FIG. 5, depending on the conversion method in the deviceRGB→CMYK converter 106, a problem may be posed if N is a constant. Thatis, in a region where colorimetric values are concentrated, distance dbecomes very small, and if N is small, the interpolation operation ismade by multiplying a few sample points by large weighting coefficients.As a result, problems such as tone jump in the device RGB color space,collapse of white balance in a low-lightness region, and the likereadily occur.

Hence, as shown in FIG. 8, when the interpolation operation is madewhile changing the number of sample points in correspondence with an L*value of the input Lab value, the aforementioned problems can beeffectively solved. Of course, even in a high-lightness region, thenumber of samples used in the interpolation operation is limited, andcolor turbidity or the like hardly occurs. Note that an example of afunction N(L*) shown in FIG. 8 is a (¼)th power function which yields128 when L*=0, and 4 when L*=100.

When the processes in steps S3 to S5 are repeated for all input Labvalues, the Lab signal can be converted into the device RGB signal.

Second Embodiment

An image processing apparatus according to the second embodiment of thepresent invention will be described below. Note that the same referencenumerals in this embodiment denote substantially the same parts as inthe first embodiment, and a detailed description thereof will beomitted.

FIG. 9 is a block diagram showing an example of the arrangement of animage processing apparatus of the second embodiment. Unlike in the imageprocessing apparatus of the first embodiment, the image processingapparatus of the second embodiment converts a signal of adevice-independent color space into a signal of a color space of theprinter 107 using an LUT as in conversion of an input signal into asignal of a device-independent color space.

An Lab→CMYK converter 803 converts an Lab signal into a signal of a CMYKcolor space depending on the printer 107 using an Lab→CMYK conversionLUT 804. A CMYK signal output from the Lab→CMYK converter 803 is sent tothe printer 107. The Lab→CMYK conversion LUT 804 is generated asfollows.

CMYK signals of sample images generated by a color patch generator 808are output to the printer 107 to obtain sample images 109.

The color patch colorimetry unit 110 measures color patches of theobtained sample images 109 to obtain Lab colorimetric values of thecolor patches. Based on the obtained Lab colorimetric values and CMYKvalues generated by the color patch generator 808, an Lab→CMYKconversion LUT generator 810 generates a CMYK→Lab conversion LUT. Basedon the generated CMYK→Lab conversion LUT, the Lab→CMYK conversion LUT804 is generated using the same method as in the first embodiment.

For example, if an Lab value is an 8-bit signal, an L* value ranges from0 to 255, and a* and b* values range from −128 to 127. If Lab grids areformed by dividing the respective ranges of Lab by 16 steps, a table ofthe Lab→CMYK conversion LUT 804 can be generated by 4913 (=17³)calculations.

In the first embodiment, the Lab color space is converted into thedevice RGB color space using the LUT, and the device RGB color space isthen converted into the CMYK color space by the arithmetic process.However, in the second embodiment, these conversion processes can bedone using a single LUT, thus improving the conversion efficiency.

Third Embodiment

An image processing apparatus according to the third embodiment of thepresent invention will be described below. Note that the same referencenumerals in this embodiment denote substantially the same parts as inthe first embodiment, and a detailed description thereof will beomitted.

FIG. 10 is a block diagram showing an example of the arrangement of animage processing apparatus of the third embodiment, and this apparatushas an arrangement for receiving an input signal of an sRGB color spacewhich will become a standard color space in the Internet. Correspondencebetween the sRGB color space and an XYZ color space has already beendefined, and the sRGB color space can be considered as adevice-independent color space. Hence, when an sRGB value is convertedinto an XYZ value or Lab value, and the Lab color space is thenconverted into a printer color space, as described above, an imageexpressed by a signal on the sRGB color space can be reproduced.

Referring to FIG. 10, an sRGB→CMYK converter 901 converts an inputsignal of the sRGB color space into a signal of the CMYK color spacewhich depends on the printer 107 using an sRGB→CMYK conversion LUT 902.A CMYK signal output from the sRGB→CMYK converter 901 is sent to theprinter 107. The sRGB→CMYK conversion LUT 902 is generated as follows.

RGB signals of sample images generated by the color patch generator 108are converted into CMYK signals depending on the printer 107 by thedevice RGB→CMYK converter 106, and the CMYK signals are output to theprinter 107, thus obtaining sample images 109.

The color patch colorimetry unit 110 measures color patches of theobtained sample images 109 to obtain Lab colorimetric values of thecolor patches. Based on the obtained Lab colorimetric values and the RGBvalues generated by the color patch generator 108, an sRGB→CMYKconversion LUT generator 908 generates a table of the sRGB→CMYKconversion LUT 902.

The process of the sRGB→CMYK conversion LUT generator 908 generates atable of the sRGB→CMYK conversion LUT 902 on the basis of CMYK valuesobtained by executing the device RGB→CMYK conversion process describedin the first embodiment for the RGB values generated by the color patchgenerator 108, and sRGB values obtained by executing Lab→XYZ andXYZ→sRGB conversion processes according to definition formulas for theLab colorimetric values. For example, if an sRGB signal is an 8-bitsignal, when sRGB grids are formed by dividing the respective ranges ofsRGB by 16 steps, a table of the sRGB→CMYK conversion LUT 902 can begenerated by 4913 (=17³) calculations.

According to each of the first to third embodiments described above, acolor conversion method which can accurately approximate the strongnonlinear output characteristics of a color printer or printing press,and can achieve accurate color reproduction can be provided. Therefore,since color space conversion that satisfactorily reflects thecharacteristics of a printer or printing press is done in adevice-independent color space, the printer or printing press canachieve accurate color reproduction for every input color spaces.

In each of the above embodiments, the Lab color space has been explainedas a device-independent color space. However, if another uniform colorspace, e.g., an Luv color space is used, the same effects can beobtained.

Fourth Embodiment

An image processing apparatus according to the fourth embodiment of thepresent invention will be described below. Since an example of thearrangement of the image processing apparatus of this embodiment issubstantially the same as that in the first embodiment, the samereference numerals denote the same parts, and a detailed descriptionthereof will be omitted. In the fourth embodiment, the conversion methodin the device RGB→CMYK converter 106 is replaced by a method differentfrom that in the first embodiment.

FIG. 11 is a flow chart showing the flow of the process in the deviceRGB→CMYK converter 106 of the fourth embodiment. An input device RGBvalue which has been normalized to [0:1] undergoes an inversion processto be converted into a CMY signal in step S1001. After that, in stepS1002 the CMY signal undergoes gamma conversion using a parameter γ,which conversion is described by:C=C^(γ)M=M^(γ)Y=Y^(γ)

In the fourth embodiment, the parameter γ in the above equations is setto be γ=1.6.

The gamma-converted CMY signal undergoes an interpolation operationusing grids (lattice points) to be converted into a CMYK signal in stepS1003, as will be described below.

Grids 1004 used in the interpolation operation process in step S1003 arelocated at the vertices of a cube on the CMY space in which each of C,M, and Y has a range of [0:1], and each grid corresponds to a CMYK valueas follows.

Grid Value Corresponding CMYK value CMY C M Y K White (W) 000 0.0 0.00.0 0.0 Red (R) 011 0.0 0.9 0.9 0.0 Yellow (Y) 001 0.0 0.0 1.0 0.0 Green(G) 101 0.9 0.0 0.9 0.0 Cyan (C) 100 1.0 0.0 0.0 0.0 Blue (B) 110 0.90.9 0.0 0.0 Magenta (M) 010 0.0 1.0 0.0 0.0 Black (Bk) 111 0.4 0.4 0.41.0

The distance between an input CMY value 1005 and each CMY grid value iscalculated, and a linear weighting operation for a corresponding CMYKvalue is made using a weighting coefficient corresponding to thedistance, thus outputting a CMYK value.

In general, an ideal toner (ink) amount cannot often be put on themedium depending on transferability, fixability, and the like of tonerin case of an electrophotography printer, or the ink permeability of themedium in case of an ink-jet printer. As a result, a secondary color(RGB) cannot often be output using a toner amount for two colors, or Bkcannot often be output using a toner amount for three or four colors.

In the fourth embodiment, assume that a toner amount only for 1.8 colorscan be used for a secondary color, and a toner amount only for 2.2colors can be used for black in the printer 107 as an output target.Hence, as shown in the table above, CMYK values corresponding to CMYvalues (0, 1, 1), (1, 0, 1), and (1, 1, 0) which indicate red, green,and blue are respectively set to be (0.0, 0.9, 0.9, 0.0), (0.9, 0.0,0.9, 0.0), and (0.9, 0.9, 0.0, 0.0). Also, a corresponding CMYK value ofblack is set to be (0.4, 0.4, 0.4, 1.0).

More generally speaking, if col2 represents a secondary color toneramount, and col4 represents a Bk toner amount, corresponding CMYK valuesof red, green, blue, and black are set as follows:

Red = (0, col2/2, col2/2, 0) Green = (col2/2, 0, col2/2, 0) Blue =(col2/2, col2/2, 0, 0) Black = ((col4-1)/3, (col4-1)/3, (col4-1)/3, 1)

That is, in the fourth embodiment, col2=1.8, and col4=2.2.

Of course, the aforementioned corresponding CMYK values and definitionsare not limited to such specific example, and can be arbitrarily set inaccordance with the device characteristics or output purposes intended.For example, if the corresponding CMYK value of black is set to be (0,0, 0, 1), pure black expressed by RGB=(0, 0, 0) can be printed usingblack toner alone, and 100% UCR can be realized.

In the fourth embodiment as well, sample images 109 are output from theprinter 107 by executing device RGB→CMYK conversion based on colorpatches output from the color patch generator 108 as in step S1 of thefirst embodiment. As in step S2, the color patch colorimetry unit 110measures the color patches of the obtained sample images 109 to obtainLab colorimetric values of the respective color patches. In the fourthembodiment, the obtained Lab colorimetric values are distributed on theLab color space, as shown in FIG. 12. As can be seen from FIG. 12, thedistribution density in a region of low L* is low, and the density in aregion of high L* is high, compared to the Lab colorimetric valuedistribution of output patches using device RGB→CMYK conversion given byequations (1) and shown in FIG. 5.

Therefore, when the color patches output based on device RGB→CMYKconversion in the fourth embodiment are used, the problems discussed inthe first embodiment, i.e., tone jump in the device RGB color space,collapse of white balance in a low-lightness region, and the like causedby dense samples in the low-lightness region, can be easily solvedwithout changing the number of sample points shown in FIG. 8 of thefirst embodiment.

In order to set the density of the colorimetric value distribution ofcolor patches independently of lightness, the parameter (γ value) ofgamma conversion can be appropriately changed in accordance with thetone characteristics of the output printer, as described in the fourthembodiment. Also, it is effective to use other calculation methods suchas a polynomial function and the like in place of gamma conversion.

Fifth Embodiment

An image processing apparatus according to the fifth embodiment of thepresent invention will be described below. Note that the same referencenumerals in the fifth embodiment denote substantially the same parts asin the first embodiment, and a detailed description thereof will beomitted.

FIG. 13 is a block diagram showing an example of the arrangement of theimage processing apparatus of the fifth embodiment. In the fifthembodiment, conversion from a signal of a device-independent color spaceinto a signal of a color space of the printer 107 is done using an LUTas in the second embodiment. Furthermore, the generation method of thatLUT will be described in detail.

In each of the above embodiments, the output device profile generationmethod has been explained. That is, the device value (e.g., CMYK)→Labconversion LUT and Lab→device value (e.g., CMYK) conversion LUTrespectively correspond to destination and source profiles of an outputdevice.

In some cases, an image that has undergone color conversion incorrespondence with the output characteristics of a printing press as atarget is printed using a copying machine or printer for the purpose ofproof (test print, proof for correction). To execute such proof, sampleimage data must be supplied to an output device used in proof to makethe device print sample images, and a profile must be generated based onthe colorimetric values of color patches of the obtained sample imagesby the method explained in each of the above embodiment. Then, an imagethat has undergone color conversion using the generated profile isprinted by the output device.

In the fifth embodiment, a profile generation process for an outputdevice used in proof will be described. Note that a profile generated inthe fifth embodiment is not limited to that for proof, but can also beused in normal output (print).

An arrangement including a CMYK o Lab converter 1201, an Lab→CMYKconverter 1202, the printer 107, a CMYK→Lab conversion LUT 1204, and anLab→CMYK conversion LUT 1205 shown in FIG. 13 is that for a generalproof system.

A CMYK signal input to the CMYK→Lab converter 1201 depends on thecharacteristics of a printing press as a target, and is converted into asignal of the Lab color space as a device-independent color space byCMYK→Lab conversion using the CMYK→Lab conversion LUT 1204 that holdscorrespondence between the CMYK color space depending on the printingpress device and the device-independent color space (Lab).

The converted Lab signal is converted into a signal of the CMYK colorspace by the Lab÷CMYK converter 1202 using the Lab→CMYK conversion LUT1205 that holds correspondence between the CMYK color space depending onthe printer 107 and the Lab color space, and the converted signal isoutput from the printer 107.

In this manner, both CMYK→Lab conversion and Lab→CMYK conversion in thefifth embodiment are implemented by loading the LUTs and makinginterpolation operations by addressing the LUTs using the input signals.

Such color matching method is executed in color matching using CRD inPostScript, or that using an ICC profile. Especially, the ICC profilecomprises a CMYK→Lab conversion LUT and Lab→CMYK conversion LUT to allowmutual conversion between a device-dependent color space anddevice-independent color space.

The Lab→CMYK conversion LUT 1205 and CMYK→Lab conversion LUT 1204 arerespectively generated by an Lab→CMYK conversion LUT generator 1211 andCMYK→Lab conversion LUT generator 1212, on the basis of colorimetricvalues by outputting CMYK color patches output from a color patchgenerator 1206 from the printer 107 and measuring the obtained sampleimages 109 by the color patch colorimetry unit 110.

The Lab→CMYK conversion LUT generator 1211 generates the Lab→CMYKconversion LUT 1205 by executing conversion from Lab colorimetric valuesof the sample images into CMYK values as in the processes of theLab→device RGB converter 104 and device RGB→CMYK converter 106 of thefirst embodiment. However, since Lab→CMYK conversion in the firstembodiment is computed based on the colorimetric values of RGB colorpatches, the colorimetric values of CMYK color patches in the fifthembodiment cannot be directly applied. Hence, in the fifth embodiment, adevice RGB→Lab conversion LUT generator 1209 is added so as to generatea pseudo colorimetric value table of RGB color patches.

On the other hand, the CMYK→Lab conversion LUT generator 1212 generatesthe CMYK→Lab conversion LUT 1204 by an interpolation operation using thecolorimetric value table of CMYK color patches.

The generated LUTs are stored in a storage unit 1213, and are set as theLab→CMYK conversion LUT 1205 to be looked up by the Lab→CMYK converter1202, or as the CMYK→Lab conversion LUT 1204 to be looked up by theCMYK→Lab converter 1201 for a proof system corresponding to anotherprinting press when they are used. Details of the operations in therespective processors shown in FIG. 13 will be described later.

FIG. 14 is a block diagram showing an example of the arrangement whenthe image processing apparatus of the fifth embodiment is implemented ona computer system, i.e., a popular computer system arrangement. Therespective processors (1201, 1202, 1206, 1209, 1211, and 1212) shown inFIG. 13 are implemented as modules of a program stored in a RAM 1303 orROM 1304, and are read out and executed by a CPU 1302. Also, the LUTs(1204 and 1205) shown in FIG. 13 are implemented as areas assured on theRAM 1303. A printer 1312 serves as the printer 107 in FIG. 13 when it iscontrolled by a printer driver, which is executed by the CPU 1302, via aprinter I/F 1311. Note that another printer present on a network 1306may be used as the printer 107 via a network I/F 1305.

The color patch colorimetry unit 110 in FIG. 13 is implemented by acolor colorimeter 1310 controlled via a serial I/F 1309, and an HDD 1308is used as the storage unit 1213. A monitor 1314 is controlled via avideo I/F 1313, and is used to display a GUI used to control respectivemodules, color patches, and the like. A keyboard 1301 and mouse 1307 areused to make user's inputs and the like via the GUI.

The operations of the processors shown in FIG. 13 will be described indetail below with reference to FIGS. 15A and 15B.

FIG. 15A is a block diagram for explaining the detailed operations ofthe patch generation/colorimetry system and CMYK→Lab conversion LUTgenerator 1212 in the fifth embodiment.

Referring to FIG. 15A, a CMYK→Lab colorimetric value correspondencetable 1401 is obtained by measuring sample images 109, which areobtained by outputting CMYK images generated by the color patchgenerator 1206 to the printer 107, by the color patch colorimetry unit110.

The color patch generator 1206 generates color patch images having,e.g., the following CMYK values.

C M Y K  0  0  0  0  32  0  0  0  64  0  0  0 . . . . . . . . . . . .224 255 255 255 255 255 255 255

In this case, each of C, M, and Y assumes values in 32-increments, and Kassumes values in 51-increments, and color patches having 9×9×9×5 colorvalues are generated. Of course, the CMYK values are not limited tothose values. A combination of those CMYK values and colorimetric values(total of 9×9×9×5 values) of patches corresponding to the CMYK valuescorresponds to the CMYK→Lab colorimetric value correspondence table1401.

The operation in the CMYK→Lab conversion LUT generator 1212 will bedescribed in detail below. The CMYK→Lab conversion LUT generator 1212comprises a CMYK grid generator 1402 and CMYK→Lab converter 1403, andgenerates the CMYK→Lab conversion LUT 1204 by looking up the CMYK→Labcolorimetric value correspondence table 1401 generated, as describedabove.

The CMYK grid generator 1402 generates combinations of CMYK values likethose generated by the color patch generator 1206 in correspondence withthe number of grids designated by the user. For example, C, M, Y, and Krespectively assume values in 32-increments, and 9×9×9×9 CMYK values aregenerated. The generated CMYK values are input to the CMYK→Lab converter1403, and are converted into Lab values by interpolation operationsusing the CMYK→Lab colorimetric value correspondence table 1401, whichis obtained by patch generation/colorimetry. The converted Lab valuesare stored as the CMYK→Lab conversion LUT 1204 on the basis ofinformation of the CMYK values generated by the CMYK grid generator1402.

FIG. 15B is a block diagram for explaining the detailed operations ofthe device RGB→Lab conversion LUT generator 1209 and Lab→CMYK conversionLUT generator 1211 in the fifth embodiment.

Referring to FIG. 15B, in the device RGB→Lab conversion LUT generator1209, a device RGB→CMYK converter 1411 converts RGB values generated bya device RGB grid generator 1410 into CMYK values, and a CMYK→Labconverter 1412 converts the CMYK values into Lab values by looking upthe CMYK→Lab colorimetric value correspondence table 1401 obtained bypatch generation/colorimetry shown in FIG. 15A. As the CMYK→Labconverter 1412, the CMYK→Lab converter 1403 as a module used in theCMYK→Lab conversion LUT generator 1212 may be commonly used.

If the device RGB grid generator 1410 generates RGB grid values:

R G B  0  0  0  0  0  32  0  0  64 . . . . . . . . . 255 255 224 255 255255Lab values corresponding to these grid values can be calculated. Thatis, Lab values equivalent to those obtained when RGB color patches aregenerated and are measured can be obtained.

The operation in the Lab→CMYK conversion LUT generator 1211 will bedescribed in detail below. The Lab→CMYK conversion LUT generator 1211comprises an Lab grid generator 1420, color space compression converter1421, Lab→device RGB converter 1422, and device RGB→CMYK converter 1423,and generates the Lab→CMYK conversion LUT 1205 by looking up a deviceRGB→Lab conversion LUT 1413 generated by the device RGB→Lab conversionLUT generator 1209. As the device RGB→CMYK converter 1423, the deviceRGB→CMYK converter 1411 as a module used in the device RGB→Labconversion LUT generator 1209 may be commonly used.

The Lab grid generator 1420 generates Lab grid values based on thenumber of grids designated by the user. For example, if an Lab value isprocessed as an 8-bit signal and the user designates 17×17×17 as thenumber of grids, L*, and a* and b* assume values in 16-increments withinthe range from 0 to 255 (*L) and the range from −128 to 127 (a*, b*),and 4913 (=17×17×17) Lab grid values are generated. The Lab valuesgenerated in this way are converted into CMYK values by the color spacecompression converter 1421, which can commonly use the color spacecompression converter 103 of the first embodiment, the Lab→device RGBconverter 1422, and the device RGB−CMYK converter 1411 which cancommonly use the device RGB−CMYK converter 106 of the first embodiment.The converted CMYK values are stored as the Lab→CMYK conversion LUT 1205together with information of the Lab grids generated by the Lab gridgenerator 1420.

FIG. 16 shows an example of a user interface (UI) used to control theprocesses of the fifth embodiment, and this UI is displayed on themonitor 1314 via the video I/F 1313 on the computer system shown in FIG.14.

A GUI 1500 shown in FIG. 16 has a view 1501 used to display a patchimage output from the color patch generator 1206. The view 1501 is usedto confirm if patch generation in the color patch generator 1206 isnormal. Buttons 1502 to 1505 are respectively, patch output,colorimetry, CMYK→Lab conversion LUT generation, and Lab→CMYK conversionLUT generation instruction buttons, and are used to instruct the startof respective processes.

Upon generating an LUT, the number of grids of the CMYK→Lab conversionLUT can be set using a grid number setting box 1507 in a CMYK→Labconversion LUT setting field 1506. This box may be prepared as, e.g., apull-down menu to allow the user to select the number of grids from9×9×9×9, 17×17×17×17, and the like.

In an Lab→CMYK conversion LUT setting field 1508, the devicecharacteristics of an output printer can be set in addition to settingof the number of grids using a grid number setting box 1509. In a devicecharacteristic individual setting field 1510, a secondary color toneramount, black toner amount, and tone correction gamma can be set asnumerical values, which are respectively set as parameters col2, col4,and γ described in the fourth embodiment, and are used as parametersupon conversion in the device RGB→CMYK converter 1411 (1423). Theseindividual setting values can be saved using a device characteristicsave button 1513, and the saved setting values can be loaded and usedagain upon depression of a device characteristic load button 1514.

When the user wants to obtain recommended values of the devicecharacteristics, he or she can designate the type of device in a devicerecommended value setting field 1511. That is, device names, andsecondary color toner amounts, black toner amounts, and tone correctiongamma values suitable for these devices are pre-stored for a pluralityof selectable devices. When the user designates the device name, anappropriate secondary color toner amount, black toner amount, and tonecorrection gamma corresponding to that device name are automaticallyset. Note that the set values may be displayed in setting columns of theindividual setting field 1510.

Note that the individual setup and device recommended value setup can beexclusively designated using check buttons. When one setup isdesignated, the other setup is grayed out, and the user can easilyrecognize the selected state.

FIGS. 17A to 17E are flow charts respectively showing processes whenoperations are instructed upon depression of the patch output button1502, colorimetry button 1503, CMYK→Lab conversion LUT generation button1504, and Lab→CMYK conversion LUT generation button 1505 on the GUI1500.

FIG. 17A is a flow chart of a patch output process (FIG. 15A) whichstarts upon depression of the patch output button 1502. This process isexecuted by the color patch generator 1206 and printer 107. After amemory area for storing the CMYK→Lab colorimetric value correspondencetable 1401 is assured in step S1601, CMYK values indicating colorpatches are generated in step S1602, and are stored in the CMYK→Labcolorimetric value correspondence table 1401 in step S1603. The printer107 as an output target is selected in the device recommended valuesetting field 1511 in step S1604, and the CMYK values of the colorpatches are output to the selected printer 107 in step S1605, thusoutputting sample images 109 from the printer 107.

After the sample images 109 are obtained, the colorimetry process of thesample images 109 (FIG. 15A) starts upon depression of the colorimetrybutton 1503. FIG. 17B is a flow chart showing the colorimetry process,which is executed by the color patch colorimetry unit 110. The sampleimages 109 output from the printer 107 are set in the color patchcolorimetry unit 110 (color colorimeter 1310), and the colorimetrybutton 1503 is then pressed. A command that instructs to startcolorimetry is sent to the color colorimeter 1310 via the serial I/F1309 in step S1611, and colorimetric values are received from the colorcolorimeter 1310 in step S1612. In step S1613, the colorimetric valuesare stored in the CMYK→Lab colorimetric value correspondence table 1401,thus completing this table 1401.

The generation processes of respective LUTs will be explained below.

FIG. 17C is a flow chart of the CMYK→Lab conversion LUT generationprocess (FIG. 15A) which starts upon depression of the CMYK→Labconversion LUT generation button 1504. This process is executed by theCMYK→Lab conversion LUT generator 1212.

The number of grids set in the grid number setting box 1507 is checkedin step S1621, and a memory area for storing the CMYK→Lab conversion LUT1204 is assured in correspondence with the number of grids in stepS1622. CMYK grid values corresponding to the number of grids aregenerated in step S1623, and are converted into Lab values in stepS1624. The Lab values are stored in the CMYK→Lab conversion LUT 1204 instep S1625, thus completing this LUT 1204.

FIGS. 17D and 17E are flow charts of the Lab→CMYK conversion LUTgeneration process (FIG. 15B) which starts upon depression of theLab→CMYK conversion LUT generation button 1505. This process is executedby the device RGB→Lab conversion LUT generator 1209 and Lab→CMYKconversion LUT generator 1211.

In step S1701, respective setting values (the number of grids, secondarytoner amount, black toner amount, and tone correction gamma value) inthe conversion LUT generator setting field 1508 are acquired and stored.In step S1702, the generation process of the device RGB→Lab conversionLUT 1413 starts.

Details of the device RGB→Lab conversion LUT generation process will bedescribed below with reference to FIG. 17E. In step S1710, a memory areafor storing the device RGB→Lab conversion LUT 1413 is assured. In stepS1711, device RGB grid values are generated and are stored in the deviceRGB→Lab conversion LUT 1413. Then, Lab values obtained via deviceRGB→CMYK conversion (step S1712) and CMYK→Lab conversion (step S1713)are stored in the device RGB→Lab conversion LUT 1413 in step S1714, thuscompleting this LUT 1413.

After the device RGB→Lab conversion LUT 1413 is obtained in step S1702,a memory area for storing the Lab→CMYK conversion LUT 1205 is assured incorrespondence with the number of grids set in the grid number settingbox 1509 in step S1703. In step S1704, Lab grid values corresponding tothe number of grids are generated. The Lab grid values are convertedinto CMYK values via color space compression (step S1705), Lab→deviceRGB conversion (step S1706), and device RGB→CMYK conversion (stepS1707), and are stored in the Lab→CMYK conversion LUT 1205 in stepS1708, thus completing this LUT 1205.

As described above, according to the fifth embodiment, since theprofiles of the output device can be appropriately generated, anappropriate proof process can be done.

Sixth Embodiment

An image processing apparatus according to the sixth embodiment of thepresent invention will be described below.

As has been explained in the first embodiment previously, it isdesirable in Lab→device RGB conversion that the colorimetric valuedistribution of RGB patches has a uniform density independently oflightness. Hence, in the first embodiment, the number of samples of Labvalues used in calculation is increased in a low-lightness region wherethe colorimetric value distribution is dense, and the number of samplesis decreased in a high-lightness region where the colorimetric valuedistribution is coarse.

To achieve the same object, the parameter γ in device RGB→CMYKconversion explained in the fourth and fifth embodiments can be adjustedto an appropriate value. As a result of adjustment, the colorimetricvalue distribution, which is dense in the low-lightness region, as shownin FIG. 5, can be obtained as the distribution independently oflightness, as shown in FIG. 12, as has already been described above.

In the fifth embodiment, the user sets the parameter γ via the GUI.However, the sixth embodiment is characterized in that the parameter γis automatically set.

FIG. 18 is a block diagram showing the arrangement of an imageprocessing apparatus of the sixth embodiment. This apparatus ischaracterized in that a colorimetric value distribution analyzer 1901for automatically setting the parameter γ by analyzing Lab colorimetricvalues corresponding to CMYK color patches is arranged in addition tothe arrangement of the fifth embodiment shown in FIG. 13. Since otherarrangements are the same as those in the fifth embodiment, the samereference numerals denote the same parts, and a detailed descriptionthereof will be omitted.

FIG. 19 is a block diagram showing the detailed arrangement of thecolorimetric value distribution analyzer 1901. Referring to FIG. 19, agray value generator 1910 generates a plurality of gray values (R=G=B),and passes them to a device RGB→CMYK converter 1911. CMYK valuesconverted by this converter are converted into Lab values by a CMYK→Labconverter 1912 on the basis of the CMYK→Lab colorimetric valuecorrespondence table 1401 (FIG. 15A) generated by the color patchcolorimetry unit 110, thus forming a gray value→L* table 1913. A γ valuecalculator 1914 calculates an appropriate γ value (to be describedlater) on the basis of the gray value→L* table 1913. The obtained γvalue is supplied to the device RGB→Lab conversion LUT generator 1209and Lab→CMYK conversion LUT generator 1211, and is set as a parameterfor the device RGB→CMYK converters 1411 and 1423.

As the device RGB→CMYK converter 1911, the device RGB→CMYK converter1411 (FIG. 15B) as a module in the RGB→CMYK conversion LUT generator1209 in the fifth embodiment may be commonly used. Likewise, as theCMYK→Lab converter 1912, the CMYK→Lab converter 1403 as a module in theCMYK→Lab conversion LUT generator 1212 (FIG. 15A) may be commonly used.

The Lab→CMYK conversion LUT generation process (FIG. 15B) in the sixthembodiment will be explained below. This process is executed by thedevice RGB→Lab conversion LUT generator 1209 and Lab→CMYK conversion LUTgenerator 1211, and its outline is the same as that shown in FIG. 17Ddescribed in the fifth embodiment. In the sixth embodiment, the deviceRGB→Lab conversion LUT generation process in step S1702 in FIG. 17D isdifferent from that in the fifth embodiment, and details of this processare shown in the flow chart of FIG. 20.

As shown in FIG. 20, the sixth embodiment is characterized in thatcolorimetric values are analyzed in step S1801 immediately after thebeginning of the Lab→CMYK conversion LUT generation process to determinea γ value, and the subsequent processes are the same as those in FIG.17E.

The γ value calculation method in the colorimetric value distributionanalyzer 1901 will be described in detail below with reference to theflow chart of FIG. 21.

Step S21

A gray signal is generated by the gray value generator 1910, and an L*value corresponding to that gray signal is obtained.

For example, the gray value generator 1910 generates a plurality of graysignals like R=G=B=0, 16, 32, . . . , 255, and the device RGB→CMYKconverter 1911 converts these gray signals into CMYK values. At thistime, γ=1.0 is set as a parameter in the device RGB→CMYK converter 1911,and col2 and col4 use values designated via the GUI of the fifthembodiment.

The converted CMYK values are converted into Lab values by the CMYK→Labconverter 1912. In this manner, Lab values corresponding to the graysignals are obtained. FIG. 22 is a graph formed by normalizing graysignals to [0:1] and plotting corresponding L* values, and this graph isthe gray value→L* table 1913.

Step S22

The correspondence (gray value→L* table 1913) between the gray signalsand L* values shown in FIG. 22 is approximated by an exponentialfunction.

L* is normalized by [0:1] by L*′=(L*−L*min)/(L*max−L*min). Thenormalized gray−L*′ curve is approximated by an exponential function bya known function fitting method to obtain a γ value.

When the γ value obtained in this way is set as parameters of the deviceRGB→CMYK converters 1411 and 1423, the distribution of Lab valuesbecomes uniform with respect to L* values in the device RGB→Labconversion LUT 1412 obtained by the device RGB→Lab conversion LUTgenerator 1209, and the Lab→CMYK conversion LUT 1205 obtained by theLab→CMYK conversion LUT generator 1211.

When the tone characteristics are corrected by a polynomial in place ofγ conversion, the same effect can be obtained if an arithmetic operationis made to fit the obtained gray signal−L* curve into the polynomial.

As described above, according to the sixth embodiment, the parameter γused in device RGB→CMYK conversion can be automatically set to be anappropriate value.

Seventh Embodiment

An image processing apparatus according to the seventh embodiment of thepresent invention will be described below.

In general, upon image formation based on CMYK, control of the ratio ofBk color, i.e., the black generation amount, is important. As a typicalcontrol method of the black generation amount, an ink (or toner) amountof black, i.e., Bk is set to be small in a low-density region, and isincreased toward a high-density region. In this way, an image which canmaintain color vividness in the low-density region, and is sharp in thehigh-density region can be obtained.

On the other hand, control parameters of device RGB→CMYK conversiondescribed in each of the above embodiment are only eight toner amountsat eight points R, G, B, C, M, Y, W, and Bk as interpolation latticepoints on the CMY space, and the γ parameter in γ conversion of C, M,and Y values serving as inputs to the CMY space on which theinterpolation operation is made. At this time, since the output CMYKvalue is calculated by a linear interpolation operation in the CMYspace, respective color components of the output CMYK value with respectto the input CMYK value simultaneously change linearly, and it isimpossible to independently control the Bk color component alone.

Hence, the seventh embodiment, as a modification of the aboveembodiments, is characterized in that independent control of a Bk colorcomponent is allowed in the device RGB→CMYK conversion process (e.g.,RGB→CMYK converters 1411 and 1423 in FIG. 15B) in each of the aboveembodiments.

The processes other than those to be described below are the same as theprocesses in each of the above embodiments.

FIG. 23 is a block diagram showing the detailed arrangement of a deviceRGB→CMYK converter in the seventh embodiment. Referring to FIG. 23, aCMY converter 2101 inverts an input RGB value by:C=1.0−RM=1.0−GY=1.0−B

A γ converter 2102 then executes γ conversion of the converted CMY valueusing an arbitrary γ value or a γ value 2111 automatically set in thesixth embodiment by:C′=C^(γ)M′=M^(γ)Y′=Y^(γ)

An interpolation arithmetic unit 2103 makes an interpolation operationof the C′M′Y′ signal using a value 2112, which is set based on toneramount limitation that defines eight lattice points in the CMY space, asin the fourth embodiment, thereby calculating a CMYK value.

A black amount adjustment unit 2104 as a characteristic feature of theseventh embodiment executes black amount adjustment using a black amountadjustment function 2113 for the calculated CMYK value. A black amountadjustment process according to the seventh embodiment will be describedin detail below.

FIG. 24 shows an example of the black amount adjustment function 2113.In FIG. 24, the abscissa plots a Bk value before conversion, and theordinate plots a Bk′ value after conversion. The black amount adjustmentfunction 2113 shown in FIG. 24 is a cubic power function given by:Bk′=Bk³

If Bk undergoes conversion based on this function, Bk is controlled todecrease the Bk generation amount in a low-density region where the Bkamount is small, and to abruptly increase the Bk generation amounttoward a high-density region. In order to control the Bk generationamount more flexibly, when a plurality of black amount adjustmentfunctions 2113 are held in the form of LUTs, it is effective to select anon-analytic function or free curve in addition to the above powerfunction.

The black amount adjustment process in the black amount adjustment unit2104 will be described in detail below with reference to the flow chartof FIG. 25.

Step S31

Bk adjustment for converting a Bk value of the CMYK value calculated bythe interpolation operation is made using the black amount adjustmentfunction 2113. That, if F(x) represents the black amount adjustmentfunction 2113, the following arithmetic operation is made:Bk_new=F(Bk)

Step S32

The Bk amount decreased in step S31 is distributed and added to other C,M, and Y values to maintain the total toner amount obtained by theinterpolation operation. The addition method is described by:C_new=C+(Bk_new−Bk)×C/(C+M+Y)M_new=M+(Bk_new−Bk)×M/(C+M+Y)Y_new=Y+(Bk_new−Bk)×Y/(C+M+Y)

Note that the decreased Bk amount need not always be distributed to C,M, and Y in correspondence with their amounts. In some cases, a valueobtained by equally dividing the decreased Bk amount may be added toeach color by:C_new=C+(Bk_new−Bk)/3M_new=M+(Bk_new−Bk)/3Y_new=Y+(Bk_new−Bk)/3

Step S33

C_new, M_new, Y_new, and Bk_new obtained in steps S31 and S32 are outputas CMYK′ after black amount adjustment.

In this way, the seventh embodiment generates a conversion condition(RGB→CMYK conversion process) for converting device-dependent color data(device-dependent RGB) specified by a plurality of color component datathat do not include a black component into device-dependent color data(CMYK) specified by a plurality of color component data that include ablack component on the basis of the tone gamma, total toner amount(print medium total amount condition), and black amount adjustmentcondition, and then generates a device RGB→Lab conversion LUT andLab→CMYK conversion LUT.

Therefore, not only the tone gamma and total toner amount but also theblack amount can be arbitrarily adjusted.

Black amount adjustment is done by the aforementioned steps. Thesemodules are implemented as a program which runs on a computer system, asshown in FIG. 14, as in the fifth embodiment, and operate incollaboration with modules corresponding to, e.g., respective processorsshown in FIG. 13.

FIG. 26 shows an example of a user interface (UI) provided by theprogram of the seventh embodiment. A UI component used to control theblack amount adjustment process is added to the GUI window shown in FIG.16 in the fifth embodiment. The UI shown in FIG. 26 will be explainedbelow.

The same reference numerals in FIG. 26 denote the same items as those inFIG. 16, and a description thereof will be omitted. FIG. 26 ischaracterized in that a black plate characteristic setting field 2301 isadded to the GUI example shown in FIG. 16. Based on the setup on thisfield 2301, a function form of the black amount adjustment function 2113used to adjust the Bk generation amount in the seventh embodiment isset.

On the black plate characteristic setting field 2301, a “set gamma”button 2302 is used to set a γ value in a functionBk′=Bk^(γ)A numerical value set in its numerical value field is set as a γ value,and is used as the black amount adjustment function 2113 in the blackamount adjustment unit 2104.

This GUI example also comprises a “free set” button 2303. Upon selectionof this button, a black amount adjustment curve panel 2304 is displayed.On this panel 2304, the function form of the black amount adjustmentfunction 2113 can be arbitrarily set as a free curve 2305. For example,as shown in FIG. 26, the user drags two predetermined marks on the graphthat represents the function to arbitrary positions, and the origin andupper right point of the graph and these marks are coupled by apredetermined method (spline function or the like), thus setting a freecurve 2305.

When the user arbitrarily sets the curve 2305 and then presses an OKbutton on the panel 2304, the set curve is set as the black amountadjustment function 2113. In this manner, when an arbitrary curve is setas the black amount adjustment function 2113, the black amountadjustment operation in step S31 is made using an LUT.

Note that “set gamma” 2302 and “free set” 2303 can be exclusivelydesignated using check buttons. When one setup is designated, the othersetup is grayed out, and the user can easily recognize the selectedstate.

Also, a recommended value of the black amount adjustment function 2113may be set in correspondence with the type of device set in the devicerecommended value setting field 1511.

As described above, according to the seventh embodiment, black amountadjustment can be made while maintaining the set total toner amount.Hence, more flexible color separation can be attained, and the profilesof an output device can be generated more appropriately.

Eighth Embodiment

The CMYK color space converted by the device RGB→CMYK converter 1411described in the above embodiment depends on an output device. The CMYKcolor space is defined by C, M, Y, and K color components correspondingto the types of color agents that the output device uses. The tonecharacteristics and a printer model formed often largely vary dependingon an output device, and such variations may disturb Lab→device RGBconversion explained in the first embodiment.

FIG. 27 shows an example of Response Curves (the relationship betweenthe CMYK value (%) and density) with respect to a CMYK input, i.e., thetone characteristics of C, M, Y, and K colors. The output of a printingpress often has the tone characteristics shown in FIG. 27. Also, asystem that simulates print tones by independently executing linear LUTconversion for each of C, M, Y, and K colors is available.

When Lab values obtained by a device RGB→Lab conversion LUT of a printermodel using the method explained in the above embodiment are plotted inassociation with the output device having the tone characteristics shownin FIG. 27, FIG. 28 is obtained. Also, when Lab values are similarlyplotted in association with an output device having linear tonecharacteristics of the relationship (Response curves) between the CMYKvalue (%) and density, FIG. 29 is obtained.

Note that the device RGB→Lab conversion LUT of the printer model isgenerated by device RGB→CMYK conversion output from the output device,and CMYK→Lab conversion according to the colorimetric result of CMYKcolor patches, as has been explained in each of the above embodiments(e.g., FIG. 15B).

As can be seen from a comparison between FIGS. 28 and 29, the volumes ofcolor cubes in a region of small L* values are different. In the colorcube (FIG. 28) of the output device having nonlinear tonecharacteristics, colors in region “A” get inside the color cube comparedto the color cube (FIG. 29) of the output device having linear tonecharacteristics. That is, color in region “A” that the printer model ofthe output device having linear tone characteristics can output cannotbe output by the printer model of the output device having nonlineartone characteristics.

In the eighth embodiment, a modification of device RGB→CMYK conversionexplained in the above embodiment will be explained to generate aprinter model from the colorimetric values of output color patches so asto obtain Lab→device RGB conversion more satisfactorily.

In the following description, a CMYK color space in which CMYK valueshave linear characteristics with respect to the density is considered asa standard color space, and a method of obtaining device RGB→CMYKconversion for outputting the color values of that color space will beexplained.

FIG. 30 is a block diagram showing an example of the arrangement of adevice RGB→CMYK converter of the eighth embodiment.

An input device RGB value is converted into a CMYK value via processesof a CMY converter 2701, γ converter 2702, and interpolation arithmeticunit 2703, as in the fourth embodiment. In the eighth embodiment, alinearity correction unit 2704 then makes conversion into a CMYK colorspace which is linear with respect to the density.

The linearity correction unit 2704 comprises linear LUT conversionindependent for each of C, M, Y, and K colors. A linearity correctionLUT 2711 is generated by a linearity correction LUT generator 2705 basedon a CMYK→Lab colorimetric value table 2712 as the colorimetric valuesof CMYK color patches. As the device RGB→CMYK converter, the process inthe seventh embodiment can be applied, and the black amount adjustmentunit can be used together.

FIG. 31 is a flow chart for explaining the process of the linearitycorrection LUT generator 2705.

When a linearity correction LUT generation process starts, Labcolorimetric values corresponding to a single color of C, M, Y, and Kare loaded for respective C, M, Y, and K colors from the CMYK→Labcolorimetric value table 2712 (S2801). For example, when colorimetricvalues corresponding to C single color are loaded, “colorimetric valueof C single color patch” can be extracted and loaded from “colorimetricvalue of color patch image” in the table below.

Colorimetric value of color patch image

C M Y K  0  0  0  0  32  0  0  0  64  0  0  0 . . . . . . . . . . . .224 255 255 255 255 255 255 255

Colorimetric value of C single color patch

C M Y K  0 0 0 0  32 0 0 0  64 0 0 0 . . . . . . . . . . . . 224 0 0 0255 0 0 0

A similar loading process is executed for the remaining M, Y, and K toobtain four correspondences C-Lab, M-Lab, Y-Lab, and K-Lab. In thesubsequent process, four linearity correction LUTs 2711 are obtainedfrom these four correspondences. However, only a method of generating alinearity correction LUT for C single color will be explained for thesake of simplicity. The remaining M, Y, and K single colors undergo thesame process as that for C single color to generate linear LUTs.

Density values are estimated from the loaded Lab values (S2802). In theeighth embodiment, the Lab values of C single color patches areconverted into C density values using an Lab→density LUT obtained inadvance, and a known interpolation operation method.

An example of the Lab→density LUT generation method will be explained.Lab values of CMYK color patches are measured, and density values arealso measured. The Lab→density LUT corresponding to C is generated bygenerating an Lab

density correspondence table based on the Lab colorimetric values anddensity values of C single color patches, and making an interpolationoperation of density values corresponding to Lab values set at equalintervals. The same process is done for M, Y, and K to generateLab→density LUTs.

Since the C density values obtained in step S2802 are obtained from Labvalues corresponding to C single color patches, they are combined withthe values of the C single color patches to obtain a C single colorvalue−C density value correspondence. Furthermore, this correspondenceis normalized to obtain a C single color value−C density value table(S2803). FIG. 32 shows this table.

Then, an inverse function of the C single color value−C density valuetable is obtained (S2804). Since conversion for converting therelationship between the C single color values and C density values intoa linear relationship is done, an inverse function can be formed byreplacing the correspondence between the C single color values and Cdensity values. FIG. 33 shows this correspondence.

Subsequently, an approximate curve of the correspondence shown in FIG.33 is obtained, and an interpolation operation is made to generate andoutput a linearity correction LUT (S2805), thus ending the process.

In the above example, the density values are estimated from the Labvalues by an interpolation operation using the Lab−density tableobtained in advance in consideration of a situation that only Labcolorimetric values are available. However, in a situation that densityvalues can be directly measured, the density values of CMYK patches maybe directly measured. In such case, the measured density values havehigher accuracy, and correction accuracy can be improved.

According to the eighth embodiment, since a printer model is formed byconverting CMYK values to change linearly with respect to the density,the color space conversion accuracy in a device-independent color spacecan be improved, and accurate color profiles can be generatedindependently of the characteristics of an output device.

Even for an output device having nonlinear tone characteristics, aprinter model that reflects characteristics which can output, e.g.,region “A” shown in FIG. 29, i.e., satisfactorily reflects a color cubethat an output device can output, can be generated. Therefore, a deviceRGB→Lab conversion table and Lab→CMYK conversion LUT can besatisfactorily generated from the printer model.

OTHER EMBODIMENTS

In the description of the above embodiments, a CMYK printer is used.However, other print agents (e.g., six color inks including C, M, Y, K,light C, and light M, or the like) may be used.

Also, in the description of the above embodiments, the RGB→CMYKconversion process is used. In place of device RGB, three colorcomponents such as C, M, and Y that do not include a black component maybe used. As can be seen from the relation between RGB and CMY in theprocess executed by the CMY converter 2101, CMY can be used in place ofRGB.

Device-independent color data is not limited to Lab data, and othercolor data such as Luv, XYZ, and the like may be used.

Furthermore, in the description of the above embodiments, the outputdevice uses four, i.e., C, M, Y, and K color agents. Alternatively, anoutput device which uses six color agents that include light C and lightM in addition to C, M, Y, and K may be used.

Note that the present invention may be applied to either a systemconstituted by a plurality of devices (e.g., a host computer, interfacedevice, reader, printer, and the like), or an apparatus consisting of asingle equipment (e.g., a copying machine, facsimile apparatus, or thelike).

The objects of the present invention are also achieved by supplying astorage medium (or recording medium), which records a program code of asoftware program that can implement the functions of the above-mentionedembodiments to the system or apparatus, and reading out and executingthe program code stored in the storage medium by a computer (or a CPU orMPU) of the system or apparatus. In this case, the program code itselfread out from the storage medium implements the functions of theabove-mentioned embodiments, and the storage medium which stores theprogram code constitutes the present invention. The functions of theabove-mentioned embodiments may be implemented not only by executing thereadout program code by the computer but also by some or all of actualprocessing operations executed by an operating system (OS) running onthe computer on the basis of an instruction of the program code.

Furthermore, the functions of the above-mentioned embodiments may beimplemented by some or all of actual processing operations executed by aCPU or the like arranged in a function extension card or a functionextension unit, which is inserted in or connected to the computer, afterthe program code read out from the storage medium is written in a memoryof the extension card or unit.

As described above, according to the present invention, a profile thatcan accurately approximate strong nonlinear output characteristics of acolor printer or printing press, and allows accurate color reproductioncan be provided.

Also, in a color printer that forms an image using a K color agent, ablack plate can be flexibly adjusted while maintaining accurate colorreproduction.

Furthermore, a conversion table that can absorb characteristicdifferences for respective color printers, and can implement accuratecolor reproduction in various color printers can be generated.

Therefore, since color space conversion that well reflects the printercharacteristics can be done in a device-independent color space,accurate printer color reproduction can be made irrespective of inputcolor spaces. Also, since color separation suitable for thecharacteristics of an output device can be easily made, the color spaceconversion accuracy in a device-independent color space can be furtherimproved.

The present invention is not limited to the above embodiments andvarious changes and modifications can be made within the spirit andscope of the present invention. Therefore, to apprise the public of thescope of the present invention, the following claims are made.

1. An image processing method comprising: an input step of inputtingcolorimetric values of color patches output from an output device; afirst generation step of generating, on the basis of the colorimetricvalues, a first conversion condition used to convert a signal on a firstcolor space which depends on a target device into a signal on a secondcolor space which is independent from a device; and a second generationstep of generating, on the basis of the colorimetric values, a secondconversion condition used to convert a signal on the second color spaceinto a signal on a third color space which depends on the output device,said second generation step including generating the second conversioncondition by: a first sub-step of converting a signal on the secondcolor space into a signal on an RGB color space; a second sub-step ofconverting a signal on the RGB color space into a signal on the thirdcolor space; and a black amount adjustment sub-step of adjusting a blackcolor value in the signal on the third color space.
 2. The methodaccording to claim 1, wherein the third color space is a CMY colorspace, and the black amount adjustment sub-step includes independentlyadjusting characteristics of a K value of a CMYK signal on the CMY colorspace.
 3. The method according to claim 2, wherein the black amountadjustment sub-step includes adjusting a K value on the basis of apredetermined black amount adjustment function, and distributing adifference between K values before and after adjustment to each of C, M,and Y color values.
 4. The method according to claim 3, wherein theblack amount adjustment function is a power function of the K value. 5.The method according to claim 4, wherein said black amount adjustmentsub-step includes adjusting the K value using an LUT on the basis of theblack amount adjustment function.
 6. The method according to claim 3,further comprising a setting step of setting the black amount adjustmentfunction.
 7. The method according to claim 2, wherein said secondsub-step further includes: converting a signal on the RGB color spaceinto a signal on the CMY color space by inverting the signal on the RGBcolor space; executing gamma conversion for the signal on the CMY colorspace; and making an interpolation operation of the signal after gammaconversion.
 8. The method according to claim 7, wherein theinterpolation operation is an arithmetic operation based on CMYK valuescorresponding to predetermined grids on the CMY color space.
 9. Themethod according to claim 8, wherein the predetermined grids are eightgrid points corresponding to R, G, B, C, M, Y, W, and Bk on the CMYcolor space.
 10. The method according to claim 9, wherein the CMYKsignal values corresponding to the predetermined grids are set inconsideration of a toner amount according to characteristics of theoutput device.
 11. The method according to claim 10, wherein saidsetting step includes setting a gamma value in gamma conversion and thetoner amount.
 12. The method according to claim 7, further comprising ananalysis step of analyzing a distribution of the colorimetric values onthe second color space, and determining a gamma value in gammaconversion on the basis of the analysis result.
 13. The method accordingto claim 1, wherein the first and second conversion conditions areembodied as lookup tables for color conversion.
 14. An image processingmethod of generating a conversion condition between a signal on adevice-independent uniform color space, and a signal on a CMY colorspace depending on a device, on the basis of colorimetric values ofcolor patches output from the device, comprising the steps of:generating the conversion condition to adjust a K value upon temporarilyconverting the signal on the uniform color space according to thecolorimetric value into a signal on an RGB color space; and convertingthe signal on the RGB color space into a CMYK signal on the CMY colorspace.
 15. The method according to claim 14, wherein the K value isadjusted based on a black amount adjustment function set based on auser's instruction.
 16. An image processing method which comprises thesteps of: inputting colorimetric values of patches, which are generatedby a target device, on the basis of device-dependent color data which isspecified by a plurality of color component data including a blackcomponent; generating, on the basis of the colorimetric values, a firstlookup table used to convert device-dependent color data, which isspecified by a plurality of color component data that do not include ablack component, into device-independent color data; and generating, bylooking up contents of the first lookup table, a second lookup tableused to convert the device-independent color data into thedevice-dependent color data, which is specified by the plurality ofcolor component data that include the black component; inputting a printagent total amount condition used to control a total amount of printagents, and a black amount adjustment condition that pertains to a blackcomponent data generation method; and performing generation of the firstand second lookup tables using a conversion condition, which is obtainedfrom the print agent total amount condition and the black amountadjustment condition, and is used to convert the device-dependent colordata which is specified by the plurality of color component data that donot include the black component into the device-dependent color datawhich is specified by the plurality of color component data that includethe black component.
 17. The method according to claim 16, wherein theprint agent total amount condition includes a total amount of a blackprint agent amount and secondary color print agents.
 18. The methodaccording to claim 16, wherein a user can arbitrarily set a conversioncurve of the black amount adjustment condition.
 19. The method accordingto claim 16, further comprising the steps of: inputting a tonecorrection gamma condition; and obtaining the conversion condition usingthe tone correction gamma condition.
 20. The method according to claim16, further comprising the steps of: generating, on the basis of thecolorimetric values, a third lookup table used to convert thedevice-dependent color data which is specified by the plurality of colorcomponent data that include the black component into thedevice-independent color data; and saving the second and third lookuptables.
 21. An image processing apparatus which comprises: firstconversion means for converting a signal on a first color space, whichdepends on a target device, into a signal on a second color space, whichis independent of a device; second conversion means for converting asignal on the second color space into a signal on a third color spacewhich depends on an output device; colorimetry means for inputtingcolorimetric values of color patches output from the output device;first generation means for generating, on the basis of the colorimetricvalues, a first conversion condition to be looked up by said firstconversion means; and second generation means for generating, on thebasis of the colorimetric values, a second conversion condition to belooked up by said second conversion means, wherein said secondgeneration means generates the second conversion condition bytemporarily converting a signal on the second color space into a signalon the RGB pace, and then converting the signal on the RGB space into asignal on the third color space.
 22. A computer-readable recordingmedium that stores, in executable form, a program for performing animage processing method that comprises: an input step of inputtingcolorimetric values of color patches output from an output device; afirst generation step of generating, on the basis of the colorimetricvalues, a first conversion condition used to convert a signal on a firstcolor space which depends on a target device into a signal on a secondcolor space which is independent from a device; and a second generationstep of generating, on the basis of the colorimetric values, a secondconversion condition used to convert a signal on the second color spaceinto a signal on a third color space which depends on the output device,said second generation step including generating the second conversioncondition by: a first sub-step of converting a signal on the secondcolor space into a signal on an RGB color space; a second sub-step ofconverting a signal on the RGB color space into a signal on the thirdcolor space; and a black amount adjustment sub-step of adjusting a blackcolor value in the signal on the third color space.
 23. An imageprocessing apparatus comprising: means for inputting colorimetric valuesof patches, which are generated by a target device, on the basis ofdevice-dependent color data which is specified by a plurality of colorcomponent data including a black component; means for generating, on thebasis of the colorimetric values, a first lookup table used to convertdevice-dependent color data, which is specified by a plurality of colorcomponent data that do not include a black component, intodevice-independent color data; means for generating, by looking upcontents of said first lookup table, a second lookup table used toconvert the device-independent color data into the device-dependentcolor data, which is specified by the plurality of color component datathat include the black component; and means for inputting a print agenttotal amount condition used to control a total amount of print agents,and a black amount adjustment condition that pertains to a blackcomponent data generation method, wherein said first and second lookuptables are generated using a conversion condition, which is obtainedfrom the print agent total amount condition and the black amountadjustment condition, and is used to convert the device-dependent colordata which is specified by the plurality of color component data that donot include the black component into the device-dependent color datawhich is specified by the plurality of color component data that includethe black component.
 24. An image processing method of generating aconversion condition between a signal on a device-independent uniformcolor space, and a signal on an RGB color space depending on a device,on the basis of colorimetric values of color patches output from thedevice, comprising the steps of: analyzing a distribution of thecolorimetric values on the uniform color space; and determining aparameter upon generating the conversion condition on the basis of theanalysis result, wherein the parameter is a gamma value used in gammaconversion for a signal on a CMY color space obtained by convening asignal on the RGB color space.
 25. A computer-readable recording mediumthat stores, in executable form, a program for performing an imageprocessing method which comprises the steps of: inputting colorimetricvalues of patches, which are generated by a target device, on the basisof device-dependent color data which is specified by a plurality ofcolor component data including a black component; generating, on thebasis of the colorimetric values, a first lookup table used to convertdevice-dependent color data, which is specified by a plurality of colorcomponent data that do not include a black component, intodevice-independent color data; and generating, by looking up contents ofthe first lookup table, a second lookup table used to convert thedevice-independent color data into the device-dependent color data,which is specified by the plurality of color component data that includethe black component; inputting a print agent total amount condition usedto control a total amount of print agents, and a black amount adjustmentcondition that pertains to a black component data generation method; andperforming generation of the first and second lookup tables using aconversion condition, which is obtained from the print agent totalamount condition and the black amount adjustment condition, and is usedto convert the device-dependent color data which is specified by theplurality of color component data that do not include the blackcomponent into the device-dependent color data which is specified by theplurality of color component data that include the black component. 26.An image processing method comprising: a colorimetry step of inputtingcolorimetric values of color patches output from an output device; afirst generation step of generating, on the basis of the colorimetricvalues, a first conversion condition used to convert a signal on a firstcolor space which depends on a target device into a signal on a secondcolor space which is independent from a device; and a second generationstep of generating, on the basis of the colorimetric values, a secondconversion condition used to convert a signal on the second color spaceinto a signal on a third color space which depends on the output device,the second generation step including the step of generating the secondconversion condition by temporarily converting a signal on the secondcolor space into a signal on an RGB color space, and then converting thesignal on the RGB color space into a signal on the third color space.27. The method according to claim 26, wherein said second generationstep includes generating an RGB conversion condition used to convert asignal on the second color space into a signal on the RGB color space.28. The method according to claim 27, wherein said colorimetry stepincludes inputting signals on the second color space as the colorimetricvalues of CMYK color patches output from the output device.
 29. Themethod according to claim 28, wherein said second generation stepincludes obtaining signals on the second color space that simulatescolorimetric values of RGB color patches by converting the signals onthe second color space as the colorimetric values of the CMYK colorpatches input in said colorimetry step on the basis of the RGBconversion condition.
 30. The method according to claim 27, wherein saidsecond generation step includes selecting several signals on the secondcolor space, which have smaller distances on the second color space,synthesizing RGB values by calculating weighted sums of RGB color valuesto which the selected signals correspond via the RGB conversioncondition, and temporarily converting the signals on the second colorspace into the synthesized RGB values.
 31. The method according to claim29, wherein a weighting coefficient used in synthesizing an RGB value insaid second generation step is a function of distance on the secondcolor space.
 32. The method according to claim 31, wherein the functionof distance is given by f(d)=1/(1+d⁴), where d is the distance on thesecond color space.
 33. The method according to claim 26, wherein, uponconverting a signal on the RGB color space into a signal on the thirdcolor space in the second generation step, an input signal on the RGBcolor space undergoes an interpolation operation on the basis of signalvalues on the third color space, which correspond to predetermined gridson the RGB color space, and are prepared in advance, so as to convertthe signal on the RGB space into the signal on the third color space.34. The method according to claim 33, wherein the predetermined gridsare eight grid points corresponding to R, G, B, C, M, Y, W, and Bk onthe RGB color space.
 35. The method according to claim 34, wherein thesignal values on the third color space, which correspond to thepredetermined grids, are set in consideration of a toner amountaccording to characteristics of the output device.
 36. The methodaccording to claim 33, wherein, upon converting a signal on the RGBcolor space into a signal on the third color space in the secondgeneration step, an input signal on the RGB color space is convertedinto a signal on the third color space by inverting the signal on theRGB color space, and the converted signal on the third color spaceundergoes gamma conversion, and then undergoes the interpolationoperation.
 37. The method according to claim 36, further comprising avariable input step of inputting a gamma value in gamma conversion andthe toner amount.
 38. The method according to claim 37, wherein saidvariable input step includes setting, when one of a plurality of devicenames is selected, a gamma value and toner amount, which are pre-storedin association with the selected device.
 39. The method according toclaim 36, further comprising an analysis step of analyzing adistribution of the colorimetric values on the second color space, anddetermining the gamma value on the basis of the analysis result.
 40. Themethod according to claim 39, wherein said analysis step includesdetermining the gamma value on the basis of a relationship between apredetermined RGB value and a signal on the second color space, which isobtained by converting the predetermined RGB value into a signal on thethird color space, and then converting the signal on the third colorspace into a signal on the second color space.
 41. The method accordingto claim 40, wherein the predetermined RGB value is a gray signal allcolor component values of which are equal to each other.
 42. The methodaccording to claim 41, wherein said analysis step includes determiningthe gamma value by approximating a relationship between the gray signaland the signal on the second color space to an exponential function. 43.The method according to claim 26, wherein the first and third colorspaces are a CMY color space, and the second color space is a uniformcolor space.
 44. The method according to claim 27, wherein the first andsecond conversion conditions and the RGB conversion condition are lookuptables for color conversion.
 45. A computer-readable recording mediumthat stores, in executable form, a program for performing an imageprocessing method comprising: a colorimetry step of inputtingcolorimetric values of color patches output from an output device; afirst generation step of generating, on the basis of the colorimetricvalues, a first conversion condition used to convert a signal on a firstcolor space which depends on a target device into a signal on a secondcolor space which is independent from a device; and a second generationstep of generating, on the basis of the colorimetric values, a secondconversion condition used to convert a signal on the second color spaceinto a signal on a third color space which depends on the output device,the second generation step including the step of generating the secondconversion condition by temporarily converting a signal on the secondcolor space into a signal on an RGB color space, and then converting thesignal on the RGB color space into a signal on the third color space.